Field of the Invention
The present invention relates to silver powders. More particularly, the present invention relates to silver powders which are suitable for applications which require electrical conductivity, such as a printed circuit board and the like.
Description of the Related Art
Electrically conductive paste is used for the production of printed circuit boards for electronic devices. The paste contains an electrically conductive powder, a binder, and a liquid organic compound (solvent). The electrically conductive powder includes a large number of fine particles containing a metal as a main component. The paste is used to print a pattern of wiring which connects electronic components to each other. After the printing, the paste is heated. By the heating, fine metal particles are sintered with fine metal particles adjacent thereto.
As a method for printing such a pattern, screen printing, gravure offset printing, photoetching, and the like are adopted.
In screen printing, the paste is placed over a screen mask on which a pattern has been formed. The screen mask is pressed against a plate by using a squeegee. By the pressing, the pattern is formed on the plate.
In gravure offset printing, grooves formed by etching or the like are filled with the paste, whereby a pattern is obtained. The pattern is transferred to a blanket, and further transferred therefrom to a plate.
In photoetching, the paste is applied to a plate. A photosensitive agent is applied to the paste. A portion of the paste is covered by a mask, and exposure to light is performed. The paste is etched, whereby a pattern is obtained.
In any of the printing methods, paste which allows for printing of narrow lines is desired. In other words, the paste needs to have good printing characteristics. Since the paste is heated, the paste also needs to have good thermal conductivity. Since the pattern is a path for electrons, the paste also needs to have good electrical conductivity.
An electrically conductive powder which is particularly suitable for the paste is silver powder. Silver powder includes a large number of silver particles. The silver particle is generally in the form of a sphere or a flake. In the paste which includes spherical silver powder, the contact area between one particle and another particle is small. Therefore, the electrical conductivity of this paste is not high. In the paste which includes flake-like silver powder, the contact area between one particle and another particle is large. Therefore, in the paste which includes flake-like silver powder, the silver is likely to form an electrically conductive network.
JP2001-49309 discloses a silver powder including a large number of spherical particles which are deposited by a reduction method.
JP2007-254845 discloses flake-like silver particles. The silver particles are formed by processing spherical particles with a bead mill.
JP2014-196527 (US2016/0001362) also discloses flake-like silver particles. The silver particles can be produced by a chemical deposition method.
The silver particles disclosed in JP2007-254845 are obtained by using a bead mill. Therefore, the particles are ill-shaped and have an uneven surface. The electrical conductivity achieved by contact between the particles is not sufficient.
The surfaces of the silver particles disclosed in JP2014-196527 are even and smooth. However, a large amount of organic matter caused by the chemical deposition method adheres to the surfaces of the silver particles.
The organic matter impairs the electrical conductivity. If the organic matter is thermally decomposed during heating of the paste, the electrical conductivity is not impaired. However, pyrolysis requires a high-temperature heating step. The high-temperature heating step causes the board to deteriorate. Furthermore, pyrolysis is normally an oxidation reaction. Therefore, when the heating step needs to be conducted in an inert atmosphere, pyrolysis is unlikely to be achieved.
Furthermore, with the silver powder disclosed in JP2007-254845 or the silver powder disclosed in JP2014-196527, the silver particles are irregularly arranged in some parts of the paste after printing. In these parts, voids are likely to occur after sintering. The voids impair the electrical conductivity of the pattern. The voids also impair the adhesion of the pattern.
It is an object of the present invention to provide a silver powder having good electrical conductivity.